1. Field of the Invention
The present invention relates to optical pickup devices for reading information signals from record media and, more particularly, to an optical pickup device operative to cause a light beam to impinge upon a record medium through an objective lens system and guide a reflected light beam coming from the record medium through the objective lens system to a photodetector, so as to obtain from the photodetector an output signal representing information read from the record medium.
2. Description of the Prior Art
In an optical disc player for optically reproducing an information signal recorded on a disc-shaped record medium, it is necessary to provide an optical pickup device for reading the information signal from a record track formed on the disc-shaped record medium.
An example of a previously proposed optical pickup device is shown in FIG. 1. The device is a semiconductor laser 1, an objective lens system 4, a photodetector 6 and other optical elements. The device is mounted on the optical disc player to be movable along a radius of a disc D which is loaded on the optical disc player and provided with, for example, a spiral record track. In the optical pick-up shown in FIG. 1, a laser light beam from the semiconductor laser 1 passes through a beam splitter 2 without being deflected thereat and then enters into a collimating lens system 3 to be collimated thereby. The laser light beam having passed through the collimating lens system 3 is focused by the objective lens system 4 to impinge upon the disc D so as to be modulated in intensity and reflected at the spiral record track on the disc D to be a reflected laser light beam.
The reflected laser light beam from the disc D passes through the objective lens system 4 and the collimating lens system 3 to the beam splitter 2 and is deflected at the beam splitter 2 so as to pass to the photodetector 6 through a light receiving lens system (a concave and cylindrical lens system) 5. The photodetector 6 detects the reflected laser light beam from the disc D to produce detection output signals varying in response to variations in the reflected light beam. The detection output signals obtained from the photodetector 6 are supplied to a signal processing circuit (not shown in the drawings) in which a reproduced information signal, focusing control signal and tracking control signal are produced. The focus control signal and tracking control signal are supplied to a lens driving unit 7 for focus control and a lens driving unit 8 for tracking control both disposed in relation to the objective lens system 4, respectively, so that a focus servo control and a tracking servo control are performed.
In the optical pickup device described above, the collimating lens system 3 by which the laser light beam from the semiconductor laser 1 is collimated to enter into the objective lens system 4 is composed of a set of concave lens element 3a and convex lens element 3b bonded together, so that the spherical aberration induced in the laser light beam passing through the collimating lens system 3 is diminished.
FIG. 2 shows schematically another example of a previously proposed optical pickup device employed in the optical disc player. The optical pickup device shown in FIG. 2 is a modification of the optical pickup device shown in FIG. 1 wherein a collimating lens system 10 is provided in place of the collimating lens system 3 shown in FIG. 1. In the optical pickup device shown in FIG. 2, the collimating lens system 10 is composed of a single convex lens element and therefore may be easily produced at reduced cost.
In the case of the previously proposed optical pickup device shown in FIG. 1 in which the collimating lens system 3 composed of a combination of the concave and convex lens elements 3a and 3b is employed so that the spherical aberration induced in the laser light beam passing through the collimating lens system 3 is diminished, a costly and time-consuming process is required for polishing four different lens surfaces including a pair of opposite outer surfaces of the concave lens element 3a and another pair of opposite outer surfaces of the convex lens element 3b, processing a peripheral portion of each of the concave lens element 3a and the convex lens element 3b, positioning both of the concave lens element 3a and the convex lens element 3b precisely in a predetermined mutual relation, and bonding the concave lens element 3a and the convex lens element 3b together precisely, in production of the collimating lens system 3. Therefore, there is a disadvantage that production cost of the collimating lens system 3 is increased.
On the other hand, in the case of the previously proposed optical pickup device shown in FIG. 2 in which the collimating lens system 10 composed of the single convex lens element so as to be easily produced at reduced production cost is employed, there is a defect that a relatively large spherical aberration is induced in the laser light beam passing through the collimating lens system 10 and therefore the laser light beam collimated by the collimating lens system 10 is apt to have a large wave front aberration. For example, FIG. 3 shows a relationship between the length (L) of an optical path from a principal plane of the single convex lens element constituting the collimating lens system 10 to a light emitting point in a semiconductor laser 1 and the wave front aberration (Ab) of the laser light beam at a spot formed on the disc D by the laser light beam, wherein Lf represents a focal distance of the single convex lens element. It is understood from this relationship that when the single convex lens element is so located that the principal plane of the single convex lens element is distant by the focal distance Lf from the light emitting point of the semiconductor laser 1 along the optical path from the principal plane of the single convex lens element to the light emitting point in the semiconductor laser 1, that is, the length (L) of the optical path is selected to be identical with the focal distance Lf, the wave front aberration (Ab) is of substantial size. In the case where the wave front aberration (Ab) of the laser light beam at the spot formed on the disc D by the laser light beam is of substantial size as described above, the optical characteristics of the optical pickup device employing the collimating lens system 10 deteriorate, for example in an information reading operation.